1. Field of the Invention
The present invention relates generally to an airfoil, and more particularly to an airfoil configuration which provides a desirable balance between structural and aerodynamic considerations relative to particular airfoil requirements.
2. Background Art
It is well known that, as a general rule, for a given wing area the wing lifting capability is reduced when the wing section cross-sectional area, which roughly correlates with wing maximum thickness, is increased. The effects of wing thickness on lift are especially detrimental when the maximum velocity at the wing upper surface is dictated by the requirement that it be no more than, for example, 1.2 times the local speed of sound, or is limited by the onset of cavitation. Such a limit on thickness is typically set for most high speed subsonic aircraft to avoid Mach shock wave drag, and this is the reason why such aircraft are equipped with relatively thin wings in comparison with thicker wings that can be used in lower speed aircraft.
However, for structural reasons, the wing of an aircraft must have a certain amount of thickness (i.e. depth dimension) so that it can properly carry the loads imposed thereon. The primary load carrying structure of a wing is customarily a spar which extends along the length of the wing (i.e. along the spanwise axis) from the inboard end to the outboard end thereof. In effect, the spar is a cantilevered beam, with the bending moments being greatest at the inboard end where it is connected to structure in the fuselage. If the thickness of the wing is to be reduced, this would necessarily require the depth of the spar to be diminished accordingly. As a general rule, the strength of a spar increases or decreases roughly in proportion to the square of its height, and the strength of the spar is generally directly proportional to spar width (i.e. the dimension parallel to the chord-wise axis of the wing).
Thus, it can be appreciated that the structural requirements of the wing are in a sense in conflict with the aerodynamic requirements. On the one hand, strength can be obtained for a given mass of spar material by increasing the thickness of the wing. However, this is generally accomplished at the expense of aerodynamic performance of the wing.
Another consideration is the amount of space that the load carrying structure (i.e. the spar) occupies in the wing. For some applications, it is desirable that the wing primary load carrying structure occupy a relatively small portion of the wing chord length, with the leading and trailing edge portions forward and rearwardly of the spar being "non-structural". As applied to the leading and trailing edge regions, the term "non-structural" refers to the negligible structural contribution these areas make to the overall wing bending and tortion stiffness and strength. Such "non-structural" areas of the wing, however, must have sufficient structural integrity to transmit local airload sheer and moment in a chord-wise direction to the spar while the spar is at its maximum spanwise bending deflection.
Since, as indicated previously, the strength of the spar in resisting bending moments is generally proportional to the square of the height, and proportional directly to the width (i.e. dimension parallel to the chord-wise axis of the wing), it can be appreciated that a percentage increment of increase in the height of the spar can permit a correspondingly greater percentage increment of decrease in the width of the spar, and yet achieve substantially the same structural strength and rigidity. Therefore, if it is desired to decrease the width of the spar so as to increase the non-structural space in the leading and trailing edge regions of the wing, one logical approach would be to increase the height of the spar where greater structural strength is needed. However, as indicated previously, since an increase in spar height would generally mean a corresponding increase in wing thickness, in general, the increase in spar height would mean a loss in aerodynamic performance.
There could be a variety of reasons for increasing the size of non structural regions of the leading and trailing edges. For example, it may be desired to house such items as antennas and sensors in the non-structural areas. Also, it may be desired to vary the camber of the wing over a greater chord length, and this would require "non-structural" components in the region of variable camber.
In view of the foregoing, it is a general object of the present invention to provide an airfoil where the primary load bearing structure has relatively high strength relative to overall weight and width dimension (i.e. the dimension parallel to the chord-wise axis of the airfoil), and yet have an airfoil which is contoured to accommodate such primary structure without unnecessarily compromising aerodynamic performance.